The present invention relates to scintillation detectors for measuring radiation at successive depths in a bore hole and more particularly to an improved type of scintillation detector having a special type of hermetic seal mounting for a high transmittance glass window to increase the efficiency and useful life of the unit.
For many years sodium iodide crystals have been provided in scintillation detectors which are used in the oil industry for deep well logging. The thallium-activated sodium iodide crystal has the ability to detect radioactivity in the geological formations surrounding a bore hole and is used to analyze such formations by being lowered into the bore hole. Temperatures up to 200.degree. C. are encountered in such situations. Before inserting the instrument in the bore hole, some users cool it to temperatures of minus 50.degree. C. or below, so that the instruments are also subjected to unusually low temperatures.
The sodium iodide crystal detects radiation by a scintillation process in which radioactive energy is converted into light energy. The crystal is enclosed in a container and provided with a glass window to allow the light signals to pass from the crystal to a photomultiplier tube or the like where they are converted into electrical signals which can be recorded and analyzed.
One of the major problems with sodium iodide scintillation crystals or other alkali metal halide scintillation crystals is the hydroscopic nature of the material. The absorption of any moisture impairs the efficiency of the crystal and can render it useless. Therefore, it is essential to maintain an effective hermetic seal throughout the temperature range in which the unit is employed.
For more than a decade prior to the present invention the best scintillation detectors for deep well logging have achieved the necessary hermetic seal by a difficult soldering operation using glass windows of a leaded glass composition which could be soldered to produce the strong reliable soldered joints which were needed. Such leaded glasses had poor light transmission characteristics. Plastic seals and other types of seals were heretofore considered unsuitable because it was not known how to provide a reliable impermeable hermetic seal while at the same time providing the needed strength, durability and temperature resistance characteristics.
The soldered construction limited the temperature at which the detectors could be used to about 150.degree. Centigrade and made it impossible to achieve optimum light transmission at a wavelength of 4200 Angstroms because of the poor light transmission characteristics of the leaded glasses. It was not known how to provide a commercially satisfactory detector without soldering the glass.
Prior to the present invention, epoxy resins were not used in scintillation detectors for deep well logging even though the ordinary epoxy adhesive bonds well to glass and steel. Such resins could even break the glass window at high temperatures because their thermal coefficient of expansion is so much higher than that of the glass. Additionally, ordinary epoxy resins do not stand up at temperatures in excess of 100.degree. C. For these reasons the ordinary epoxy resins would be unsuitable for use in deep well scintillation detectors and would not maintain the necessary hermetic seal for an adequate period of time during normal use in deep well logging operations.